1. Field of the Invention
The invention relates generally to valves for cylinders, and more particularly to a double valve for emptying a cylinder independently of the pressure within the cylinder while providing pressure monitoring ports.
2. Background and Description of Prior Art
Cylinders having a single valve on the top are frequently used to store or transport hazardous liquids and gases. The cylinder is partially emptied by opening the valve and allowing the internal pressure within the cylinder to expel the contents through the valve. The emptying process continues only as long as the cylinder pressure is positive with respect to the external pressure. The emptying process halts when the pressure is no longer positive, and the remaining cylinder contents cannot be readily emptied.
In the past it was difficult to know how empty the cylinder really was. The cylinder was weighed before and during the emptying process and the net weight provided information, accurate to perhaps 10% or 15% of full scale, as to how empty the cylinder was. It was desired, however, to have more accurate information as to the degree of emptiness of the cylinder. In many applications, the inability to substantially completely empty and thus use 100% of the gas in the cylinder is unacceptable. Tungsten hexafloride, for example, costs approximately $15,000 per cylinder, and leaving 10% of this gas behind in an "almost-empty" cylinder would cost $1,500. Other gases used in the semiconductor industry such as hydrogen fluoride or boron trichloride are toxic to humans. Another industrial gas, dichlorosilane, is not only toxic, but explodes at atmospheric conditions. Obviously it is important to completely empty a cylinder of such gases to avoid a dangerous situation in case someone inadvertently opens a not-quite "empty" cylinder containing these materials.
Past solutions for further emptying the cylinder include heating the cylinder to increase the internal pressure. However heating is dangerous and can produce an explosion, especially when the cylinder contains hazardous substances. Heating can also cause the cylinder walls to be chemically attacked by the cylinder contents. Further, heating the cylinder may require heating of all connecting plumbing to avoid condensation. Heating also disturbs the calibration of any flow meters connected to the system to monitor the emptying process. A related solution includes connecting the outlet side of the valve to a vacuum system, thereby making the pressure in the cylinder effectively more positive.
A more practical solution is to pressurize the cylinder with an inert gas such as nitrogen or argon from an external source. In some cases the inert gas atomizes the cylinder contents and volatilizes or blows the contents out the valve. Such aerosol techniques are not suitable, however, for dispensing materials in highly controlled applications such as in the semiconductor industry where impurity content must be tightly controlled.
U.S. Pat. No. 3,925,033 to Mayo (1975) discloses a system for bubbling a combustible gas through a liquid in a cylinder to increase the mixture of gases which burn with a hotter flame. Mayo's device uses separate inlet and outlet valves and uses considerable plumbing to accomplish the mixing.
Prior art apparatus for emptying a cylinder have relatively obstructed flow passages through which the gas being emptied must pass. Such obstructed passages, coupled with "dead spaces" that are typically found in prior art valves, make it difficult to purge the cylinder system. The lack of effective purging can result in contamination when trapped ambient air mixes with the gas to be emptied. Contamination is intolerable where pure gases are being handled, such as in the semiconductor industry. Further, contamination of hazardous gases can result in toxic conditions or explosions.
3. Objectives of the Invention
It is an objective of the invention to provide a compact and safe valve mechanism to facilitate the dispensing of or emptying of the contents of a cylinder regardless of internal pressure in the cylinder.
It is another objective to provide a valve mechanism that eliminates dead space and obstructed passageways, thereby facilitating the complete purging of the cylinder emptying system without recourse to vacuum pumps or heating.
It is a further objective to provide a valve mechanism that facilitates the accurate monitoring of differential pressuring to enable an accurate measurement of the quantity of gas still in the cylinder being emptied.
It is a still further objective to provide a valve mechanism that exhibits positive valve closure at all times, and provides a pressure release function as well.